Despite the enormous efforts in academia and in pharmaceutical industry, Ab vaccination, y- and b-secretase inhibitors are still far from clinical use. Drugs that ameliorate AD phenotypes by interfering cholesterol metabolism have been also suggested, but the rationale for using those is poorly defined and in all of the cases the molecular mechanisms which account for their beneficial effects are poorly understood. Therefore there remains a great need for other strategies of lowering Ab. Nuclear Liver X receptors (LXRa and LXRb) are transcription factors that control the expression of genes involved in cholesterol metabolism. We have found that activated LXR regulate metabolic pathways of brain cholesterol intra- and extracellular transport that influence APP processing, b-amyloid deposition and its clearance from brain. Moreover, we and others have shown, that treatments of neuronal cell lines and primary neurons with natural or synthetic LXR ligands decrease Ab secretion, and that in vivo treatment of young AD model mice with a synthetic LXR ligand, increased the expression of LXR responsive genes in CNS and decreased soluble b-amyloid levels in their brains. We have also found that the application of LXR ligands inhibited secretion of inflammatory cytokines and increased neuronal survival following Ab or LPS treatment. The ultimate beneficial outcome of LXR ligand treatment on AD phenotype in vivo is a combination of effects mediated by genes expressed in neurons and astrocytes influencing Ab generation, b-amyloid formation and clearance, and by genes expressed in microglia which are tightly related to both - Ab clearance and inflammation. The data therefore substantiate the design of a drug discovery study for identification of LXR activators/agonists and their systematic evaluation in well established in vitro and in vivo model systems. In Specific aim 1, the activated status of LXR will be evaluated by their ability to increase dramatically the transcription of ABCA1 - a major LXR response gene. Thus, the increased amount of ABCA1 mRNA following in vitro application of a given compound (altogether >50,000 for screening) will serve as a reporter for LXR activation. Compounds that meet the criteria for LXR activation will be further screened for their ability to reduce Ab production in vitro. "Hits" with dual stimulatory effect on ABCA1 expression and apoE secretion, ability to reduce Ab and to facilitate cholesterol efflux will be further characterized in LXR knockout cell based assays. In addition, potent compounds will be characterized for their inhibitory effects on cytokine expression and regulatory effects on transcriptional activity of genes with undesired effects on fatty acids and triglyceride synthesis. In Specific Aim 2, the two most potent compounds will be chosen for initial in vivo tests in PS1APP transgenic mice. PUBLIC HEALTH RELEVANCE: Despite the enormous efforts in academia and in pharmaceutical industry, drugs based on the recent progress in our understanding for Alzheimer disease are still not available for clinical use. Brain cholesterol metabolism and its relation to different aspects of the disease pathogenesis substantiates the development of new therapeutic approaches based on complex regulatory networks controlled by transcription factors and their responsive genes. We are proposing a drug screening approach for identification of natural or synthetic compounds that activate transcription factors called Nuclear Liver X receptors (LXRa and LXRb) and therefore lead to upregulation and increased expression of genes involved in cholesterol metabolism. This drug discovery study is based on data generated in our and other laboratories which confirm that LXR controlled cholesterol transporters and lipoproteins in brain are involved in b-amyloid aggregation, deposition and clearance. Therefore, LXR activators can be used for lowering Ab inhibition of inflammation and improvement of behavioral deficits.